Preconditioning the human myocardium by simulated ischemia: studies on theearly and delayed protection

Background: There are data supporting the existence of ischemic preconditio ning in man. This study investigated the most effective preconditioning pro tocol for the human myocardium and whether the second window of ischemic pr econditioning (24 h) is as protective as the first window (less than or equ al to 2 h). Methods and results: Right atrial appendages (n=6/group) obtain ed during coronary bypass surgery were prepared and superfused with normoxi c and normothermic Krebs-Henseleit solution. After 30 min stabilisation, mu scles were subjected to various preconditioning protocols followed by 90 mi n ischemia and 120 min reperfusion. At the end of each protocol, the leakag e of creatinine kinase (CK, U/g wet wt) and the reduction of MTT to insolub le formazan dye (OD/mg wet wt), an index of cell viability, were measured. In study 1, preconditioning was induced by 2, 3, 5 and 10 min of ischemia f ollowed by 5 min reperfusion. In study 2, 1-4 cycles of 2 or 5 min ischemia -5 min reperfusion were applied. In study 3, preconditioning was induced by 5 min ischemia-5 min reperfusion followed by 1, 2, 3 or 4 h reperfusion be fore the subsequent 90 min ischemia. In study 4, preconditioning with 5 min ischemia followed by 5 min reperfusion either immediately preceded 30 or 9 0 min ischemia/120 min reperfusion or was applied 24 h before. In study 1 a nd 2, optimal protection was achieved with 5 min or two cycles of 2 min pre conditioning ischemia (CK=3.06+/-0.31 and 2.89+/-0.02; MTT=0.56+/-0.05 and 0.47+/-0.09, respectively vs. CK=5.56+/-0.51 and MTT=0.18+/-0.04 in ischemi a alone group; P<0.05). In study 3, protection was observed 2 h after preco nditioning (CK=3.43+/-0.22 and MTT=0.46+/-0.09; P<0.01 vs. ischemia alone g roup) but it was lost beyond 2 h (CK=6.30+/-0.56 and MTT=0.16+/-0.02 after 3 h; P=NS vs. ischemia alone group). In study 4, protection was observed 24 h following preconditioning when the atrial specimens were exposed Co 30 m in ischemia (CK=2.96+/-0.38 and MTT=0.61+/-0.01 vs. CK=4.56+/-0.26 and MTT= 0.43+/-0.02 in ischemia alone group, P<0.05); however, when the period of i schemia was extended to 90 min the beneficial effect of preconditioning was lost (CK=10.28+/-0.5 and MTT=0.11+/-0.09 vs. CK=9.56+/-0.62 and MTT=0.104/-0.05 in ischemia alone group, P=NS). Conclusions: In the isolated human m yocardium maximal protection induced by preconditioning is achieved by a to tal 4-5 min ischemic stimulus, an effect that is lost beyond 2 h of its app lication. Two windows of protection were identified, the first (less than o r equal to 2 h) being more potent than the second (24 h). (C) 2000 Publishe d by Elsevier Science B.V. All rights reserved.